Hybrid image-pupil optical reformatter

ABSTRACT

A hybrid image-pupil optical reformatter and method for optional use with a spectrometer is disclosed, which performs beam slicing in pupil space and stacks replicas of the input source generated from the pupil beam slices in image space. The optical reformatter comprises a collimator which receives an input light and produces a collimated beam; a first optical element which receives the collimated beam, redirects portions of the collimated beam back toward the collimator as reimaged beams and permits portions of the collimated beam to pass; a second optical element which receives the reimaging beams and redirects the reimaging beams back toward the collimator and the first optical element; to form an output beam comprising the portions of the collimated beams that are not redirected toward the collimator by the first optical element. Also disclosed is the use of the reformatter for reformatting the input light of a spectrometer system, and the use of the reformatter as part of a spectrometer device.

RELATED APPLICATION

This application claims priority from U.S. Provisional Application No.62/105,928 filed Jan. 21, 2015, the content of which is hereinincorporated by reference.

FIELD OF THE INVENTION

This invention relates to the field of optical reformatters and morespecifically relates to improved reformatter apparatus and methods forimproving the performance of an optical system, such as improving thespectral resolution of an optical spectrometer.

BACKGROUND

Optical reformatters tend to be implemented for receiving input imagesand light beams and producing reshaped output images and light beamswhich are better suited for measurement by an optical system such as anoptical spectrograph or a detector or detector array, or for furtherprocessing by a light processing system. Specifically, opticalreformatters are useful for preparing and configuring light that passesto an optical spectrograph from the output of a light source such as anoptical fiber, a bundle of optical fibers, a telescope, an image relay,or a physical aperture such as an input slit.

By way of background, conventional optical spectrographs include a smallinput aperture, typically a slit. The input aperture could alternativelybe a circular pinhole, an optical fiber, or other input means; however,for the sake of brevity, the input aperture will hereinafter be referredto as a slit. An input light may be a converging or diverging beam oflight projected towards the slit, or may be some other light sourceplaced such that a portion of the light passes through the slit. In atypical optical spectrograph, light passing through the slit isprojected onto a lens or mirror which collimates the light to form abeam of substantially parallel light rays. In a typical opticalspectrograph, a dispersive element, such as, a prism, a transmissiongrating, or reflection grating, bends the collimated beams by differingamounts, depending on the wavelength of the light, thereby producing aspectrally dispersed light beam. Often, a camera lens or mirror bringsthese spectrally dispersed beams into focus on an array detector, suchas a charge-coupled device (CCD) detector, or some other single elementor multi-element detector located at the final focal plane, and whichmay measure the focused spectrum and record the light intensities of thevarious wavelengths.

In a typical optical spectrograph, the collimating lens (or mirror) andthe camera lens (or mirror) act as an image relay, to create images ofthe light passing through the slit on the detector, such as a CCDdetector, with the images displaced laterally depending on thewavelength of the light. The spectral resolution of an opticalspectrograph, a quantitative description of its ability to detect andmeasure narrow spectral features such as absorption or emission lines,can be dependent upon various characteristics of the spectrograph. Suchcharacteristics may include the dispersing element, for example theprism, transmission grating, or reflection grating; the focal lengths ofthe collimating lens (or mirror) and camera lens (or mirror); and thewidth of the slit along the dispersive axis. For a particular disperserand camera lens, the resolution of the spectrograph can be increased bynarrowing the width of the input slit, which causes each image of thelight passing through the slit (depending on the wavelength of thelight) and onto a detector, to subtend a smaller section of thedetector, allowing adjacent spectral elements to be more easilydistinguished from each other.

By narrowing the width of the input slit, less light passestherethrough, which can reduce the quality of any measurements due to areduction in the signal-to-noise ratio. In some applications, such asastronomical spectroscopy, high-speed biomedical spectroscopy,high-resolution spectroscopy, or Raman spectroscopy, this loss ofefficiency can be a limiting factor in the performance of the opticalspectrograph. A device which increases the amount of light that can passthrough the slit by compressing an image of an input beam of light alongthe dispersive axis (i.e. horizontally), while substantially maintaininglight intensity or flux density, would be advantageous in the field ofoptical spectrography even if the spot image is compressed along thedispersive axis at the expense of expansion along a perpendicular axis(i.e. vertically).

A person of skill will understand that the terms “horizontal”,“vertical” and other such terms used throughout this description, suchas, “above” and “below”, are used for the sake of explaining variousembodiments of the invention, and that such terms are not intended to belimiting of the present invention.

A person of skill will also understand that while the term component isusually used to refer to a specific item such as a lens or a mirror, andthe term element is usually used to refer to a group of components thatshare a common functional purpose, it is also possible to have anelement made up of a single component, or a single component whichfunctions as multiple elements. For example, in the case of an opticalcomponent with multiple reflective or refractive surfaces such as a lenswith a reflective coating, the lens could have the function of oneelement and the reflective coating could have the function of adifferent element. Similarly, a curved mirror could both redirect alight beam and change the divergence of the light beam, therebyproviding the function of multiple elements in the same component.

A person of skill will also understand that the focused image producedby focusing a collimated beam may be referred to as a spot or a spotimage, and that a light source does not need to be a focused spot imagein order to be collimated. An image refers to the light field spatialdistribution at the focal plane of a lens or mirror wherein thewavefront concavity changes direction, while image-space refers to anyspace in the light field where the wavefronts are substantially notplanar. A pupil refers to a lateral cross section of a light fieldwherein the wavefronts are substantially planar, and pupil-spacetherefore refers to any location where the wavefronts are substantiallyplanar.

Optical reformatters can be useful to receive an input beam and/or inputimage and produce output beams and/or output images that are bettermatched to spectrometer input slits. An optical slicer is one type ofoptical reformatter in which portions of the beam or image are dividedup and redirected or repositioned.

An optical slicer comprising transparent prisms and plates to slice aninput beam can have deficiencies because it may produce a reformattedimage at a slit that is tilted along the optical axis, and additionallythe slicing of an optical beam can occur along the hypotenuse of a 45°prism, which can result in focal point degradation due to differentsections of the sliced image being located at different focal positions.The performance of such slicers can also depend on the absorptioncoefficient and index of refraction of the prism material used (whichare both wavelength dependent). These deficiencies can limit the use ofsuch slicers in broadband optical devices.

There also exists other optical slicers which are image slicers, such asthe Bowen-Walraven slicer or optical fiber spot-to-line converters,which operate entirely in the image space. Some such image slicersgenerally do not preserve the spatial image information and aretherefore unable to resolve spectral information from different portionsof a source image independently. These reformatters are also challengingto implement in a commercially feasible way, can be large in size, andcan result in reduced or inefficient implementation of a variety ofsystems. These slicers often produce multiple copies of the slit imagewhich can result in wasted space on the detector due to gaps between theslices at the final focal plane, which may add noise to the signal andthus decrease the quality of the output data, limit the number ofspectra (or spectral orders) that can fit on the detector, and reducethe efficiency of the detector readout because of the spectrum beingspread over a larger detector area. Optical slicers using optical fiberbundles to allow the extended (often round) image of an input source tobe formed into a narrow slit image can also cause degradation of theoutput f-ratio and the total performance to be inefficient. Existingslicer devices almost uniformly suffer this decreased efficiency andoutput f-ratio, which is a clear limitation of slicer design andimplementation. Also, optical fiber bundles tend to be inefficient forlight collection due to gaps between the individual fibers and spacetaken up by the individual fiber claddings.

More recently, new pupil reformatter designs, and the use of pupilreformatters to improve the spectral resolution of a spectrograph, havebeen disclosed. These slicer-based reformatters operate entirely inpupil space, slicing and then anamorphically expanding a collimatedbeam. This approach is useful when spatial image information needs to bepreserved, such as with push-broom hyperspectral imaging, multi-fiberinputs, etc., but pupil beam divergence can be problematic with largerinput sources, and optical system complexity increases with an increasein the number of slices created.

The present invention differs from existing reformatter designs in thatit operates partially in pupil space and partially in image space. Assuch, it is referred to throughout this application as a hybridimage-pupil optical reformatter and embodiments of the present inventioncan be described as a hybrid slicer or hybrid reformatter, whichoperates partially in pupil space and partially in image space. Thisapproach has advantages over traditional optical slicers including ininstances when a larger number of slices are desired since operating areformatter partially in pupil space and partially in image space, asdisclosed in this invention, tends to be characterized by back-and-forthoptical beam paths through a collimator which limits the beam fromspreading out. In embodiments of the present invention, larger slicingfactors may be achieved with fewer components, reduced beam divergencelosses, and less demanding alignment tolerances, and the number ofslices tends to be relatively independent of the optical complexity,with a preferred number of slices being approximately equal to the ratioof the input beam width to the output beam width. Embodiments of thepresent design also tend to more easily handle larger input spot sizesand/or faster input beams (small f-ratios) than traditional opticalslicers.

The pupil beam in the present invention tends to get narrower withoutgetting taller, and the pupil slices disclosed in embodiments of thepresent invention tend to overlap. This is in contrast to most pupilreformatters in which the pupil beam gets both narrower and taller, andthe pupil slices generally do not overlap. Further, many other opticalreformatters use ‘explicit’ expansion as part of the reformatting whilethe expansion is ‘implicit’ in some embodiments disclosed in the presentinvention.

SUMMARY OF THE INVENTION

In an aspect of the present invention there is provided an opticalreformatter for generating an output beam comprising a collimator whichreceives an input light and produces a first collimated beam; a firstoptical element which redirects one or more portions of the firstcollimated beam toward the collimator to produce one or more reimagingbeams and which allows one or more portions of the first collimated beamto pass the first optical element to form part of the output beam; and asecond optical element which redirects some or all of the reimagingbeams toward the collimator to produce additional collimated beams, suchthat portions of the additional collimated beams also form parts of theoutput beam.

In some embodiments of the present invention, the one or more portionsof the first collimated beam that form part of the output beam may passthe first optical element without any further redirection. In otherembodiments, the input light may be the output of an optical fiber, animage relay, or a physical aperture.

The collimator may be singlet lens, complex lens, single mirror, orother optical element which collimates a diverging beam and focuses acollimated beam. Further, the first collimated beam and additionalcollimated beams may be substantially or perfectly collimated. Stillfurther, the first optical element and the second optical element mayeach comprise one or more mirrors.

In some embodiments, the one or more portions of the first collimatedbeam redirected toward the collimator may be located at the extremitiesof the first collimated beam. In other embodiments, the one or moreportions of the first collimated beam redirected toward the collimatormay be redirected non-parallel to the first collimated beam; or the oneor more portions of the first collimated beam redirected toward thecollimator may be redirected non-parallel to one another.

The reimaging beams may produce focused images at positions non-incidentto the input light, and the second optical element may be positioned toredirect the one or more reimaging beams without blocking the pathbetween the input light and the collimator. The second optical elementmay also be positioned at the location where the reimaging beams producefocused images.

In further embodiments, the one or more portions of the additionalcollimated beams may be redirected toward the collimator by the firstoptical element, to produce additional reimaging beams; and theadditional reimaging beams may be redirected toward the collimator bythe second optical element, to produce further additional collimatedbeams, such that one or more portions of the further additionalcollimated beams also pass the first optical element to form part of theoutput beam. In still further embodiments, such redirection of theadditional collimated beams and additional reimaging beams may beiterative and repetitive in nature.

In some embodiments, substantially all of the light energy received fromthe input light may be contained in the output beam. Additionally, theportions of the first collimated beam and additional collimated beamswhich form the output beam may be substantially overlapping andpropagating in substantially the same direction. The output beam mayalso be narrower in dimension than the first collimated beam.

The optical reformatter may also comprise an additional optical elementto redirect the output beam after it passes by the first opticalelement. Alternatively, or in the addition, the optical reformatter mayalso comprise a focusing element to focus the output beam onto an inputof a spectrometer. In some embodiments, the focusing element may be arod lens, a cylindrical lens, a cylindrical mirror, or one or morecylindrical or toroidal lenses or mirrors.

In a further aspect of the present invention, the optical reformattermay also an optical element to expand the output beam along a firstdimension to produce an expanded beam; a dispersive element tospectrally disperse the expanded beam along the first dimension toproduce a spectrally dispersed beam; a focusing element to focus thespectrally dispersed beam to produce a focused spectrum; and a detectorto receive and measure the focused spectrum.

In a still further aspect of the present invention, there is provided amethod of generating an output beam comprising collimating an inputlight through a collimator to produce a first collimated beam;redirecting one or more portions of the first collimated beam backthrough the collimator to produce one or more reimaging beams;redirecting some or all of the reimaging beams through the collimator toproduce additional collimated beams; and forming an output beam from theadditional collimated beams and portions of the first collimated beamswhich are not redirected back through the collimator.

In some embodiments, the portions of the additional collimated beams mayalso be redirected back through the collimator to produce additionalreimaging beams, and some or all of the additional reimaging beams maybe redirected through the collimator to produce further collimatedbeams, such that output beam may include portions of the furtheradditional collimated beams. In further embodiments, the redirection isiterative.

The redirected beams and beam portions may be redirected to produce anoutput beam formed from substantially overlapping beams and beamportions, propagating in substantially the same direction, and withsubstantially all of the light energy in the input light. The redirectedbeams and beam portions may also be redirected to produce an output beamthat is narrower in a first dimension relative to the first collimatedbeam. In some embodiments, the method may also comprise focusing theoutput beam onto the input of a spectrometer. In some embodiments, theoutput beam may be expanded along a first dimension to produce anexpanded beam; the expanded beam may be spectrally dispersed along thefirst dimension to produce a spectrally dispersed beam; the dispersedbeam may be focused to produce a focused spectrum, and the focused beammay be measured.

BRIEF DESCRIPTION OF FIGURES

For a better understanding of embodiments of the system and methodsdescribed herein, and to show more clearly how they may be carried intoeffect, reference will be made by way of example, to the accompanyingdrawings in which:

FIG. 1A shows an isometric view of an embodiment of a hybrid image-pupiloptical reformatter;

FIG. 1B shows an isometric view of the embodiment of the hybridimage-pupil optical reformatter of FIG. 1A in use with or as part of anoptical spectrometer;

FIG. 2 illustrates the slicing and redirection of collimated beams in anembodiment of a hybrid image-pupil optical reformatter, shown as crosssections of the reformatter at the pupil mirrors.

FIG. 3 shows the shapes of the pupil beams and focused images that mayexist at various points in the operation of embodiments of a hybridimage-pupil optical reformatter such as shown in FIGS. 1A and 1B.

DESCRIPTION

It will be appreciated that, numerous specific details have provided fora thorough understanding of the exemplary embodiments described herein.However, it will be understood by those of ordinary skill in the artthat the embodiments described herein may be practiced without thesespecific details. In other instances, well-known methods, procedures andcomponents have not been described in detail so as not to obscure theembodiments described herein. Furthermore, this description is not to beconsidered so that it may limit the scope of the embodiments describedherein in any way, but rather as merely describing the implementation ofthe various embodiments described herein.

In the descriptions and figures below, references to “top”, “bottom”,“left”, “right”, “horizontal”, “vertical” etc. are used for convenienceand clarity only. They are in no way meant to limit the possibleorientations of the various optical components and structures, ratherthey are used to describe and illustrate the relative orientations ofcertain elements within the design disclosed in the present application.The use of the term “collimated” in this application shall include bothperfectly collimated and substantially collimated.

In the descriptions and figures below, optical elements such as mirrorsand lenses are used to illustrate the invention. It may be possible toachieve the same result using different optical elements, or by using adesign in which reflections are replaced with transmissions, ortransmission are replaced with reflections, to achieve the desiredeffect on the optical signals.

With reference to FIG. 1A, an embodiment of the hybrid image-pupiloptical reformatter is shown. For reference, the hybrid image-pupiloptical reformatter is depicted as part of a dispersive spectrometersystem in FIG. 1B. The system of FIG. 1B may all be contained within asingle physical enclosure, or may be split between several physicalenclosures with appropriate optical coupling between them. Directions inFIGS. 1A and 1B are described herein as “horizontal” wherein theorientation is substantially parallel to the dispersion axis of thedisperser and “vertical” wherein the orientation is substantiallyperpendicular to the dispersion axis of the disperser, although personsof skill will appreciate that the system can be configured with otherorientations.

In the embodiment of FIG. 1A, light source 110 a produces beam 112,shown as a diverging beam, which is collimated by collimating element113 into a first substantially collimated beam 114. Light source 110 amay be the output of an optical fiber, a bundle of multiple opticalfibers, an image relay, a physical aperture, or some other source.Several types of optical elements could be used to form collimatingelement 113, including for example, a singlet lens, doublet lens, acomplex lens, a single mirror or a complex mirror, or other opticalelement which collimates a diverging beam (and which focuses acollimated beam, based on the principle of optical reversibility).

In the embodiment shown, collimated beam 114 arrives at optical element115, shown as comprising a pair of mirrors 115 a and 115 b whichredirect portions of beam 114 by reflection. In other embodiments,optical element 115 may comprise other optical components to redirectportions of beam 114. These mirrors, also referred to as pupil mirrorsbecause they operate on a collimated pupil beam, could be configuredwith straight edges incident on the beam profile and could for examplebe two vertical D-mirrors although persons of skill will appreciate thatother optical elements and configurations of optical elements could beused. In the embodiment depicted mirrors 115 a and 115 b are flat andseparated by a small gap such that a portion of collimated beam 114passes between the gap, forming part of collimated output beam 116.Persons of skill will appreciate that the term “passes” or “pass” couldrefer to movement across, movement through, movement by, or othersimilar motion. Part of beam 114 reflects from mirror 115 a and isredirected toward collimating element 113. Collimating element 113converts the redirected collimated beam into a reimaging beam when thecollimated beam passes back through it, which converges and comes to afocus near (ie. non-coincident with) image 110 a. This focal point maylie upon optical element 111 which is shown as comprising a pair ofreflective mirrors but may comprise other optical components to redirecta beam. This refocused image tends to be the same size as input source110 a and tends to have a lower light intensity. In the embodimentshown, mirror 115 a is tilted vertically (i.e. the vertical axis istilted around the horizontal axis) such that the reflected beam portionis not parallel to beam 114. This change to the angle at which thereflected beam portion passes through the collimator causes therefocused spot image 110 b to be displaced vertically from image 110 a,landing on flat mirror 111 a which is located above image 110 a.Similarly, another part of beam 114 reflects from mirror 115 b, passesback through optical element 113, and comes to a focus near image 110 a.However, mirror 115 b is tilted such that the reflected and refocusedspot image 110 c is displaced vertically from image 110 a in theopposite direction of image 110 b, landing on flat mirror 111 b which islocated below image 110 a. Mirrors 111 may also be referred to as imagemirrors because they tend to operate on a focused image. These imagemirrors could for example, be two horizontal D-mirrors although personsof skill will appreciate that other optical elements and configurationsof optical elements could be used. In some embodiments, one of the imagemirrors may be positioned in the source image focal plane just above thesource image, while the other may be positioned in the source imagefocal plane just below the source image, such that the source imageenters the slicer by passing between the image mirrors. In otherembodiments the mirrors, or a single mirror, may be a one-way mirrorplaced in front of the input source such that it allows light to enterfrom the input source while still reflecting the reimaging beams.

Images 110 b and 110 c reflect from their respective mirrors backtowards optical element 113, where they are recollimated into additionalcollimated beams similar to and substantially coincident with beam 114,but slightly tilted and laterally offset. In the embodiment shown,mirror 111 a is tilted horizontally such that the redirected reimagingbeam is directed toward collimator 113 at a different horizontal anglethan diverging beam 112, such that the additional collimated beamproduced is shifted toward the center of optical element 115, relativeto the redirected collimated beam portion produced by mirror 115 b.Similarly, mirror 111 b is tilted horizontally such that the additionalcollimated beam produced from the redirected reimaging beam is shiftedtoward the center of optical element 115, relative to the redirectedcollimated beam portion produced by mirror 115 a. These additionalcollimated beams encounter mirrors 115 a and 115 b, and a portion ofeach passes through the gap between the pupil mirrors and are added tooutput beam 116, while other portions are reflected back through opticalelement 113 to form spot images 110 d and 110 e on mirrors 111 a and 111b. Depending on the spacing and angular tilt of mirrors 115 a, 115 b,111 a, and 111 b, the number of multiple reflections and spot images 110may be two, three, four, five, or any larger number, and the number ofspot images, multiple reflections, and beam portions passing between thepupil mirrors may or may not be equal. In other embodiments, opticalelements 115 and 111 may each comprise a single mirror and there mayonly be a single reflection from each and a single additional spot imageproduced. In some embodiments, preferred for some implementations, thebeam portions remaining on the final reflection pass fully by mirrors115 a and 115 b without any portions being reflected. In alternativeembodiments, there may be reflections that result in optical element 115reflecting all of the light and not allowing any of the light to formpart of output beam 116 until reflected back again later. Also, inalternative embodiments optical element 111 may only redirect some ofthe reimaging beams or some portions of the reimaging beams and not allof the reimaging beams as shown.

Collimated beam 116 may therefore also comprise a plurality ofsubstantially similar and spatially coincident beams or beam portions,each corresponding to one of the spot images 110, and together thosebeams and beam portions form the output beam of the optical reformatter.Each of the beam portions which form the output beam may have a tallnarrow profile, similar in height to the first collimated beam andnarrower in width, and each with a slightly different vertical tilt. Thevertical tilt tends to create a slight vertical divergence in the outputbeam. In some embodiments, preferred for some implementations,reformatter output beam 116 comprises substantially all of the lightintensity (light energy) contained in the input light, with only smalllosses due to the reflective or transmissive efficiency of the opticalcomponents in the reformatter. In some embodiments, an additionaloptical element may redirect output beam 116 in order to change thelayout of the optical system or to couple more conveniently orefficiently with a downstream optical element.

FIG. 2 further illustrates the slicing and redirection of the collimatedbeams and additional beam portions, as shown at the pupil mirrors, in anembodiment of a hybrid image-pupil reformatter. FIG. 2a shows firstcollimated beam 203 being received by pupil mirrors 201 and 202,corresponding to mirrors 115 a and 115 b in FIG. 1. FIG. 2b shows theleft (204) and right (205) portions of the beam being redirected back tothe collimator (not shown) while the middle portion has passed throughthe gap between the pupil mirrors to form part of the output beam. FIG.2c shows the additional collimated beam 206 produced from beam portion204 after it has been reimaged, redirected by the corresponding imagemirror (not shown), and recollimated by the collimator (not shown). Notethat in the embodiment shown, beam 206 has been flipped and shiftedtoward the middle of the pupil mirrors 201 and 202, and shifteddownwards relative to beam 204. A portion of beam 206 will pass throughthe gap between the pupil mirrors to also form part of the output beam,while the other portion of beam 206 will be reflected by mirror 202.Similarly, FIG. 2d shows additional collimated beam 207 produced frombeam 205 after it has been reimaged, redirected by the correspondingimage mirror (not shown), and recollimated by the collimator (notshown). Part of beam 207 will pass between pupil mirrors 201 and 202,joining the output beam, while the other portion of beam 207 will bereflected by mirror 201. FIG. 2e shows beam 208, the portion of beam 206which is reflected by the pupil mirror while the remainder of beam 206passes through the gap between the pupil mirrors. Finally, FIG. 2f showsbeam 209 produced from beam portion 208 after reimaging and redirectionby the image mirror (not shown) and recollimation by the collimator (notshown). This process may continue until all of the light has beenshifted into the gap between the mirrors and passed through to form partof the output beam. It can be seen that in an embodiment preferred forsome applications, the number of iterations will be equal to the widthof the first collimated beam divided by the separation distance of thepupil mirrors.

Returning to FIG. 1B, an example embodiment is shown in which anoptional image relay with one or more curved lenses or mirrors is usedto reimage a light source such as a fiber output onto the reformatterlight source focal plane 110 a. This may be advantageous when using afiber-fed source because with the image relay, the fiber cladding orjacketing or ferrule will tend not to interfere with the image mirrors,and the f-ratio within the slicer section can be altered from thef-ratio exiting the fiber, for instance to slow down the f-ratio toreduce aberrations in the collimating lens. In the image relay of FIG.1B, input aperture 101 transmits a diverging beam 102 with a broadbandspectral profile. Persons of skill will appreciate that input aperture101 could be implemented for example, using an optical fiber, a pinhole,or a light source, however other input sources would also be suitable.Diverging beam 102 is refocused by optical element 103, which isdepicted in the embodiment of FIG. 1B as a singlet lens. Optical element103 could be implemented using numerous types of optical elements, forexample a doublet achromat lens, a complex lens, a single concavemirror, or a complex mirror system. Optical element 103 focuses the beaminto a converging beam 104 which forms an image 110 a of input aperture101.

FIG. 1B also shows how the output of the reformatter could be directedtoward the input of a spectrometer, or how the reformatter could bedirectly incorporated into a spectrometer. Persons of skill willappreciate that there are several different ways to send the reformatteroutput beam into the dispersive spectrometer section of the system. Inthe embodiment shown, collimated reformatter output beam 116 passesthrough focusing element 117, which may be a rod lens, cylindrical lens,cylindrical mirror, or any other optical element which will tend tofocus the beam portions along the horizontal axis and not along thevertical axis. In the embodiment shown, the light in the beam portionstherefore tends to focused into a tall narrow slit-like image (relativeto input aperture 101 which may have had a circular appearance) atintermediate focal plane 118. A physical slit or light baffle may beplaced at focal plane 118 to restrict the light passing through, blockscattered light, make the slit image even narrower (at the cost ofreduced light intensity), or all of the horizontally focused light maybe permitted to pass through this focal plane. A spectrometer could alsobe placed with its input aperture at focal plane 118.

Continuing the description of the hybrid image-pupil optical slicerbeing used as part of a dispersive spectrometer as shown in FIG. 1B,upon passing through focal plane 118, the plurality of beam portionsforms beam 120, which tends to look like a diverging beam, for examplef/5 vertically and f/5 horizontally. This diverging beam is collimatedby optical element 121 to create collimated beam 122, which has beenexpanded in the dispersive direction relative to output beam 116.Expansion could also be implemented using an expander element to divergeand recollimate the beam, for example a convex lens and concave lens, ora convex mirror and concave mirror, instead of components 117 and 121which cause the beam to converge through a focus before forming adiverging beam which is subsequently recollimated. This expansion of thepupil beam contributes to narrowing of the refocused image produced fromthat beam. Expanded collimated beam 122 reflects from flat fold mirror123 to dispersion element 124, which may be a diffraction grating,prism, grism, or any other spectrally dispersive element. The disperser124 creates a spectrally dispersed beam 125 comprising a plurality ofmonochromatic collimated beams, where the horizontal angle of each beamis dependent upon its wavelength. Focusing element 126, comprising forexample, a simple or complex lens, or a simple or complex mirror, orcombination thereof, focuses these dispersed beams on to focal planedetector 127 on detector system 128, which may be a CCD device, CMOSdevice, InGaAs sensor, linear photodiode array, photographic film,single-pixel photodiode or photomultiplier tube, or any otherlight-detecting apparatus. The measured intensities of each sensorelement in detector system 128 provide a measurement of the spectraldistribution of the original light beam passing through aperture 101.The optical sequence from focal plane 118 to detector system 128 issimilar to many other dispersive spectrometer designs, however, unlikeother dispersive spectrometer designs, the beam reformatting method ofthe hybrid image-pupil optical slicer implemented by elements 101through 117, reshapes the input source 101 into a tall narrow image atfocal plane 118 to provide higher spectral resolution without losinglight at a narrow slit.

Persons of skill will appreciate that in some cases it may beadvantageous to make use of a dispersive spectrometer with additionaloptical reformatting, a dispersive spectrometer with additional beamexpansion and/or compression along one or more axes, or some otherdispersive spectrometer design.

FIG. 3 illustrates the shapes of the pupil beams and focused images thatmay exist at various points in the embodiment of FIG. 1A and FIG. 1B.The first collimated pupil beam 114 is shown as a round beam withrelatively uniform intensity. The reformatted output beam 116 is shownwith a width corresponding to the gap between mirrors 115 a and 115 b,and with superimposed beam portions contributed by the first collimatedbeam and the additional collimated beams. Overlapping D-shaped portionsof the additional collimated beams which pass optical element 115combine such that some portions of the output beam have a greater lightintensity than others. Expanded output beam 122 is also shown. FIG. 3also shows input source image 110 a, replicated sources 110 a-e at thefocus of collimating element 113 (which is also the source image focalplane and the position of mirrors 111), and the refocused camera imageat the focal plane of the camera image (127). Note that the intensity ofthe replicated sources decreases with each pass between the first andsecond optical elements because additional beam portions pass opticalelement 115 to form part of the output beam on each pass, and less ofthe light is reflected back.

In some embodiments, there may be a different number of pupil mirrors(for example, 1 or 3 instead of a pair), a different number of imagemirrors, or even unequal numbers of pupil and image mirrors. Someembodiments may also include only a single pupil slicing mirror and asingle image mirror, which would tend to provide only two slices. Insuch an alternative, light would tend to pass around the image mirrorrather than through a gap between the mirrors. Furthermore, in otherembodiments the design may be configured such that not all portions ofall the reflected pupil beams are later reflected back into the pupil bythe image mirrors.

The design could tend to be described as a first pass design ininstances where beams are only redirected between the pupil mirrors andimage mirrors one time. As the number of passes increases (i.e.increased iterations) and the portion of the collimated beams whichpasses by the image mirrors to form part of the output beam on each passdecreases, the narrowing of the output beam relative to the firstcollimated beam can be larger which may be advantageous, for example byenabling the output beam to be expanded by a larger factor withoutbecoming larger than the first collimated beam. In some systems thenumber of iterative passes could be quite large. However, as the numberof passes increases, the light intensity will decrease due to reflectiveand transmissive losses. A compromise between these two factors willdetermine the most appropriate number of passes for a givenimplementation.

In the present invention, the collimated pupil beam tends to be slicedinto separate sub-beams, like the traditional optical slicers, whichtends to have no effect on the refocused spot, but the tilt in thoseslicing mirrors tends to shift the refocused spot images vertically, sothe “stacking” is done in image space instead of in pupil space. Assuch, the different collimated slices are all superimposed on top ofeach other in pupil space instead of being stacked vertically. Eachcollimated slice will tend to have a different vertical angle, and thetotal bundle of slice beams will tend to have greater verticaldivergence than any single beam. In practice, this vertical divergenceis similar to the horizontal divergence exhibited after the input ispassed through the slit, such that the downstream optics can be round orsquare instead of highly rectangular. The tall skinny pupil consistingof the superimposed slices can then be focused horizontally to make anintermediate virtual slit image which can be passed through a physicalslit if desired (to reduce scattered light leakage) before going to adispersive spectrometer backend. The horizontal focusing can be achievedusing a cylindrical lens, to create a virtual slit image in which theoriginal image spots are ‘blurred’ together into a single column oflight. Persons of skill will appreciate that the focusing could also bedone in other ways, such as using a spherical lens, although such aspherical lens may require an impractical f-ratio to implement.

It is also possible to direct the reformatted pupil beam in the presentinvention, without undergoing a horizontal focusing, to the input of adispersive spectrometer specially designed to handle this type of input.There may also be applications in which the reformatted output in thepresent invention, either collimated or focused, could be used as theinput to other optical devices aside from only dispersive spectrometers.

Persons of skill will appreciate that in some embodiments certain of theoptical elements which are depicted in FIG. 1 could be replaced withalternative elements which provide similar functionality via a differentmethod, or combine the functionality of two or more original elements.By way of example, the lens-based transmissive reimager (103) could bereplaced with a catadioptric system or fully-reflective reimagingsystem. By way of further example, the collimator (113) could beimplemented with an off-axis paraboloid mirror instead of a lens. By wayof further example, the pupil mirrors (115 a and 115 b) could bereplaced by a single mirror with a slit or hole cut into it. By way ofyet further example, the collimator (113) and pupil mirrors (115 a and115 b) could be combined into a single element consisting of atransmissive lens with one surface fabricated with different angles andpartially coated with a reflective coating to act as a mirror ormirrors.

While the present invention can be used with any device that tends touse light as an input, as previously noted, one example of the use ofthe optical slicer described herein may be in the field of spectroscopy.A general spectrometer is a device that disperses light such that theintensity value of light as a function of wavelength can be recorded ona detector. For readings that require a higher spectral resolution, anarrower slit tends to be needed in a direct relationship to spectralresolution and typically, a narrow slit will provide a reduction in thelight intensity received at the detector or sensor focal plane of thegeneral spectrometer device. Positioning an optical slicer in front ofthe input of a general spectrometer device, possibly in combination withsome form of implicit or explicit beam expansion, can tend to produce aninput into the general spectrometer device slit having an increasedlight intensity value as compared to a slit without an optical slicer,by the factor of the slicing factor, over the area of the slit, tendingto provide increased spectral resolution without sacrificing lightsignal intensity.

A subset of spectroscopy is interferometric spectroscopy; the definingfeature of interferometric spectrometers is that the dispersing elementused is not a grating or a prism. Rather, the dispersion is achievedanother way, such as by taking the Fourier transform of the patterngenerated by two interfering beams. The slicer not only increasesbrightness of the output, but also allows large improvements in thecontrast of the interference fringes, as well as signal-to-noise ratio.

A further subset of interferometric spectrometry as pertains to medicalimaging is Optical Coherence Tomography (OCT), a technique that uses aninterferometric spectrometer to make an image. A slicer will improve thethroughput, as well as the fringe contrast, of the OCT device; theresult is that the slicer can improve the depth penetration possiblewith OCT systems, speeding imaging time and increasing the value of thecaptured image. An optical slicer can be included at the input to theOCT device.

An optical slicer can be used in a subset of OCT called Fourier domainOCT (FD-OCT), and more specifically in a specific implementation FD-OCTcalled Spectral Domain OCT (SD-OCT). An SD-OCT instrument is aninterferometric spectrometer with a dispersive spectrometer to recordthe signal. An optical slicer can be included at the input to thedispersive spectrometer right before the dispersive beam element in acollimated beam path.

A further application of the slicer is in the field of miniaturespectroscopy, particularly as it pertains to Raman spectroscopy. CurrentRaman spectrometers have been implemented that are miniaturized tohandheld scale. As the slicer can be used to increase throughput in anysystem wherein light is used as the input source, a miniaturizedembodiment of the slicer can be used in conjunction with miniaturizedspectrometers, like the Raman spectrometer, to increase spectralresolution, increase output signal strength, and decrease scan time. Anoptical slicer can be included at the input to the Raman spectroscopydevice.

The present invention has been described with regard to specificembodiments. However, it will be obvious to persons skilled in the artthat a number of variants and modifications can be made withoutdeparting from the scope of the invention as described herein.

The invention claimed is:
 1. A spectrometer comprising: an opticalreformatter for generating an output beam, comprising: a collimatorwhich receives an input light and produces a first collimated beam; afirst optical element which (i) redirects one or more portions of thefirst collimated beam so they are received by the collimator whichproduces one or more reimaging beams; and (ii) allows one or moreportions of the first collimated beam to pass by the first opticalelement to form part of the output beam; a second optical element whichredirects one or more portions of the reimaging beams so they arereceived by the collimator which produces additional collimated beams;wherein one or more portions of the additional collimated beams alsoform part of the output beam; an optical element to expand the outputbeam along a first dimension to produce an expanded beam; a dispersiveelement to spectrally disperse the expanded beam along the firstdimension to produce a spectrally dispersed beam; a focusing element toFocus the spectrally dispersed beam to produce a focused spectrum; and adetector to receive and measure the focused spectrum.
 2. A method ofgenerating an output beam comprising: collimating an input light using acollimator to produce a first collimated beam; using an optical elementto redirect one or more portions of the first collimated beam so theyare received by the collimator which produces one or more reimagingbeams; using an optical element to redirect one or more of the reimagingbeams so they are received by the collimator which produces additionalcollimated beams; and forming an output beam from the additionalcollimated beams and portions of the first collimated beams which arenot redirected so they are received by the collimator.
 3. The method ofclaim 2 wherein portions of the additional collimated beams are alsoredirected so they are received by the collimator which producesadditional reimaging beams, and one or more of the additional reimagingbeams are redirected so they are received by the collimator whichproduces further collimated beams; wherein the output beam includesportions of the further additional collimated beams.
 4. The method ofclaim 3 wherein the redirection is iterative.
 5. The method of claim 2wherein the redirected beams and beam portions are redirected to producean output beam formed from substantially overlapping beams and beamportions, propagating in substantially the same direction, and withsubstantially all of the light energy in the input light.
 6. The methodof claim 2 wherein the redirected beams and beam portions are redirectedto produce an output beam that is narrower in a first dimension relativeto the first collimated beam.
 7. The method of claim 2 furthercomprising focusing the output beam onto the input of a spectrometer. 8.The method of claim 2 further comprising: expanding the output beamalong a first dimension to produce an expanded beam; spectrallydispersing the expanded beam along the first dimension to produce aspectrally dispersed beam; focusing the spectrally dispersed beam toproduce a focused spectrum, and; measuring the focused spectrum.